1. Field of the Invention
The present invention relates to a fluid dynamic pressure bearing unit in which both loads in a radial direction and a thrust direction of the rotating shaft are supported in a non-contact condition by fluid dynamic pressure of lubricating oil supplied in a gap between a bearing and a rotating shaft during the rotating of the rotating shaft. The fluid dynamic pressure bearing unit of the present invention is preferably used as a bearing unit of a spindle motor used as a driving motor for various kinds of information devices such as driving units in disc drive devices which read and write information from and to a magnetic disc or an optical disc, polygon motors in laser printers, etc.
2. Description of Related Art
With respect to such spindle motors for information devices, in order to increase the recording density and to increase the speed for transferring data, rotating performance at high speed and with high precision is desired. A non-contact type of fluid dynamic pressure bearing has widely been adopted as a bearing which supports a rotating shaft in recent years for motors which are superior in these characteristics. In the fluid dynamic pressure bearing, an oil film is formed by supplying lubricating oil in a small gap between the rotating shaft and the bearing, and the oil film is compressed by the fluid dynamic pressure of the lubricating oil generated due to rotation of the rotating shaft, so that the rotating shaft is supported with high rigidity by the compressed oil film. In the fluid dynamic pressure bearing, the fluid dynamic pressure is effectively generated by a groove (a fluid dynamic pressure groove) for generation of the fluid dynamic pressure formed on one of the sliding surfaces of the rotating shaft and the bearing.
The bearings for spindle motors are constructed such that a load in a thrust direction is supported in addition to the usual load in a radial direction. A flange, which is integrally formed in the rotating shaft, is slidably faced to an end surface of the bearing, so that the thrust load on the rotating shaft is supported on the end surface of the bearing through the flange. The above fluid dynamic pressure is generated in both the thrust direction and the radial direction. That is, a thrust fluid dynamic pressure groove is formed on either the flange of the rotating shaft or the end surface of the bearing, which face each other, and a radial fluid dynamic pressure groove is formed on either an inner peripheral surface of the bearing or an outer peripheral surface of the rotating shaft which faces the inner peripheral surface. In the fluid dynamic pressure grooves, the shape and depth are designed so that an oil film of lubricating oil is compressed with rotation of the rotating shaft, and for example, the shape of the groove may be generally wedge-shaped in which width and depth are smaller and narrower in a rotating direction of the rotating shaft, etc.
It is preferable that the gap (bearing gap) between the rotating shaft and the bearing in which an oil film is formed be narrower since oil pressure is easily increased and the bearing rigidity is improved. In other words, bearing performance is decided by management or control of the bearing gap. For example, Japanese Unexamined Patent Application Publication No. 2005-188753 discloses an invention in which bearing gaps at a radial side and a thrust side are justified by setting squareness and flatness of the bearing end surface to specific values.
When the bearing gap is increased, since fluid dynamic pressure is not increased sufficiently by lowering oil film pressure and a tendency toward low bearing rigidity is observed. Furthermore, when a reduction in the viscosity of the lubricating oil is promoted by a temperature increase (about 60° C.) with continuous operation of the spindle motor, the bearing rigidity is remarkably insufficient. Therefore, as described in the above publication, it is effective that the bearing gap be designed to the specified values or less and be as small as possible. However, when the bearing gap is too small, there is a problem in that the motor is difficult to start due to increased viscous friction of the lubricating oil, that is, the stationary rotating shaft is difficult to rotate. In particular, at a thrust side at which the flange tends to tightly come into contact with the bearing end surface by gravity when the rotating shaft stops, such a starting problem easily occurs, and in the case in which the environmental temperature in which it is used is low, for example, below 0° C., there is a problem in that the motor cannot be started due to increased viscous friction of the lubricating oil. In addition, in a spindle motor which uses battery power at the same time in a portable device such as a notebook-type PC (personal computer), starting torque is small. In such spindle motor, miniaturization of the bearing gap is limited from the viewpoint of the starting.
In contrast, in the case in which the inclining height of the rotating shaft is large, squareness accuracy of the bearing end surface which receives thrust load from the flange of the rotating shaft is low. Therefore the rotating shaft is inclined and comes into contact with the inner peripheral surface by coming into contact the flange with the bearing end surface, and increasing friction interferes with the starting. Furthermore, when the flange and the bearing are made from different materials, the gap is further narrowed by thermal expansion difference thereof, and the above various problems due to the narrow gap conspicuously occur.